It is frequently necessary to augment the convective stack effect to increase the draw in a chimney or the venting of a vent conduit. Static ventilators are known having no moving parts which use ambient wind in a venturi effect to increase the draft. Also known are dynamic or turbine-type ventilators which also use the ambient wind, but to move a turbine or fan that operates as an axial-input radial-output blower to increase the draft. It is also, of course, known to provide a simple motor-driven blower on a stack so as to increase the draft to any desired extent.
All of these above-described devices exist in many different sizes and of constructions that give various venting capacities. It is therefore necessary for the purchaser to determine exactly how much increased venting is needed before buying such a unit. Miscalculation requires replacement of the entire unit with another. The user normally errs on the side of a smaller and cheaper unit, so that any change usually requires scrapping the original purchase and replacing it with a heavier duty device.